ci: remove redundant pnpm setup from Playwright test jobs

The test scripts (test_playwright_marimo.sh, test_playwright_wasm_marimo.sh,
test_playwright_jupyter.sh) already run their own `pnpm install` / `npm install`
in packages/buckaroo-js-core/. The CI-level pnpm setup + workspace-wide
`pnpm install --frozen-lockfile` was redundant. Node.js is still set up
since Playwright requires it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: paddymul

.github/workflows/checks.yml

@@ -250,18 +250,8 @@ jobs:
           enable-cache: true
           python-version: "3.13"
           prune-cache: false
-      - name: Setup pnpm
-        uses: pnpm/action-setup@v4
-        with:
-          version: 9.10.0
-      - name: Setup Node.js with pnpm cache
+      - name: Setup Node.js
         uses: actions/setup-node@v4
-        with:
-          cache: 'pnpm'
-          cache-dependency-path: 'packages/pnpm-lock.yaml'
-      - name: Install pnpm dependencies
-        working-directory: packages
-        run: pnpm install --frozen-lockfile
       - name: Download build artifacts
         uses: actions/download-artifact@v4
         with:
@@ -290,18 +280,8 @@ jobs:
           enable-cache: true
           python-version: "3.13"
           prune-cache: false
-      - name: Setup pnpm
-        uses: pnpm/action-setup@v4
-        with:
-          version: 9.10.0
-      - name: Setup Node.js with pnpm cache
+      - name: Setup Node.js
         uses: actions/setup-node@v4
-        with:
-          cache: 'pnpm'
-          cache-dependency-path: 'packages/pnpm-lock.yaml'
-      - name: Install pnpm dependencies
-        working-directory: packages
-        run: pnpm install --frozen-lockfile
       - name: Download build artifacts
         uses: actions/download-artifact@v4
         with:
@@ -364,18 +344,8 @@ jobs:
           enable-cache: true
           python-version: "3.13"
           prune-cache: false
-      - name: Setup pnpm
-        uses: pnpm/action-setup@v4
-        with:
-          version: 9.10.0
-      - name: Setup Node.js with pnpm cache
+      - name: Setup Node.js
         uses: actions/setup-node@v4
-        with:
-          cache: 'pnpm'
-          cache-dependency-path: 'packages/pnpm-lock.yaml'
-      - name: Install pnpm dependencies
-        working-directory: packages
-        run: pnpm install --frozen-lockfile
       - name: Download build artifacts
         uses: actions/download-artifact@v4
         with: